A. Some women after becoming pregnant will ovulate again weeks later, and this second egg may be fertilized, resulting in fraternal twins of different gestational ages. That's rare enough, says University of Virginia biologist Robert J. Huskey.

Rarer still is "heteropaternal superfecundation," or fraternal twins with different fathers. After a woman in China gave birth to different sex twins, the husband accused her of having sex with another man and refused responsibility for the children. Blood tests and enzyme markers proved he was indeed the father of the girl though not the boy.

"Confronted with the evidence, the mother admitted to having sex with a second man within a three-day period at the time of conception," reported the Journal of Forensic Science.

Topping even this, a New York woman who went to the doc was mistakenly implanted with two fertilized eggs, then gave birth to "twin" boys: one white, one black! One of the eggs was hers, but the other belonged to a different woman and couple. Result: four parents, one pregnancy, and totally genetically unrelated twin-born babies.

Q.What have to be two of the all-time worst disguises in the annals of crime and crime-fighting? ­R. W. Mettetal

A: Superman's and Batman's. Start with the fact that each of us can distinguish thousands, maybe even tens of thousands of different faces, says John P. Dworetzky in Psychology. Even identical twins won't fool you once you get to know them.

And people are such keen face-spotters– way beyond the current capabilities of any programmed computer– they can recognize an old friend they haven't seen in 20 years, in spite of wrinkles, sags, graying hair.

"In truth, Superman had no real hope of disguising himself from those who knew him by combing his hair slightly

differently and putting on a pair of horn-rimmed glasses."

And even where the eye might get taken in, the ear probably won't. "Together, the eyes and ears are formidable detectives. Even Bruce Wayne's friends meeting Batman for the first time would take one look at his mouth, hear his voice, and ask, `Bruce, why are you dressed as a bat?'"

Q.What has been the biggest flying creature ever? Alongside this one, an eagle seems but a sparrow. ­S. S. Transport

A. "Quetzalcoatlus," after the feathered Aztec deity, was a flying reptile (pterosaur) of 65 million years ago, its remains found in Texas in the 1970s, says Chris McGowan in Diatoms to Dinosaurs: The Size and Scale of Living Things.

Quetzalcoatlus weighed maybe 200 pounds and had a 40-foot wingspan, big as a light twin-engined airplane. "This seems an enormous weight to get into the air, but people weighing this much can hang-glide," McGowan says.

Riding thermals and soaring may have kept it aloft, but how ever to take off? It could have launched from a high place, but what if it landed? Research by J.H. Marden suggested flight muscles plus powerful anaerobic fibers could have provided ample takeoff power. "But it's still hard to imagine a 40-foot- wingspan animal flapping fast enough for takeoff," he notes.

Which raises lifestyle questions, says McGowan. If it lived by the sea, maybe it fed on the wing like today's seabirds. But remains have been found far inland. If a carrion feeder, maybe it launched from atop huge decomposing sauropod carcasses. "Quetzalcoatlus raises more questions than it answers, but we can be quite sure that fly it DID."

A. Beyond the human spotter and operator manually inputting the correct yard line into the system, a special camera mount holds the cameras and encodes their movements, such as tilt, pan, zoom and focus, so the system knows what every camera is doing in real-time, says Howstuffworks.com.

As the cameras pan, their perspectives must be recalculated at 30 frames/second. More: Since the field crests in the middle for water runoff, the virtual line must follow this curve. The system must sense (via color, etc.) when players, referees or the ball crosses the line so the line isn't painted right over them on TV screens.

Again, all of this must be done for all cameras at once, moving at different rates and from different views. "It takes a tractor-trailer rig of equipment, including eight computers and at least four people for the task," says the website.